In a plurality of applications in technical systems parts or components can be subjected to stresses which alternate or vary over time, of a mechanical or thermal nature for example. In such cases individual parts can for example be subjected to direct mechanical stresses through the occurrence of compression or tensile forces. A time-varying thermal stress of this type arises on the other hand for example for the parts or components in a turbine system, especially in a steam turbine, when the steam turbine is started up or shut down.
This means that when the steam turbine is started up the turbine parts are increasingly heated up from a cold initial state until a comparatively high temperature level has been set during operation in accordance with design specifications. When the steam turbine is shut down on the other hand the parts start off in a comparatively hot initial state and are cooled down ever further until all components have reached the temperature of their surroundings. During this warming-up and cooling-down phase a temperature difference arises in a few parts between the surface directly exposed to the heating or cooling medium and the interior of the part concerned. These types of temperature difference can lead to thermal stresses in the part and thus to a direct load on the part.
The occurrence of mechanical or thermal loads of the type stated in the parts can lead at a microscopic level to rearrangement processes in the crystal structure of the parts or such like. The result of these types of time-varying loads is thus what is usually known as material fatigue or exhaustion of the part concerned, which is accompanied by a successive deterioration or detrimental effect on the material properties such as hardness or load capacity for example. As the fatigue state of the part or the associated detrimental effect on its material properties increase, the part concerned may possibly no longer meet the specific design criteria such as load capacity or similar for example, so that as a result of the ongoing fatigue of the component concerned with time-varying loading, its lifetime or future usability are restricted. For the parts subjected to an alternating or time-variant stress there is thus usually provision, taking into account the fatigue or material exhaustion occurring, for replacement of the component concerned in good time or also for other suitable maintenance within a predetermined maintenance interval.
To avoid unnecessary shutdowns of the relevant technical systems and the associated high maintenance costs and such like, or to keep said costs especially low, planning of maintenance intervals and such like adapted to the fatigue or exhaustion state of the part under particular load is usually provided. In order to achieve this in a particularly targeted fashion, provision is made for determining an approximate value of a parameter characteristic of the fatigue state of the part concerned. To determine a characteristic parameter of this type, the stress cycles of the part, also referred to as the load cycles, are evaluated. To do this the progress over time of the stress of the part concerned is monitored by continuously recording a measured value characteristic for it.
The disadvantage of the concept described is however that the measured values included for the determination of the characteristic parameter for the exhaustion state, particularly with stress cycles, which, depending on the use of the relevant part, can only be computed over s significant period of for example months or even years, can only be made available in a timely manner under some conditions.